1. Field of the invention
The invention concerns a photonic concentrator for connecting subscriber terminals to a telecommunication network using asynchronous time-division multiplexing. This type of multiplexing enables all kinds of information to be transmitted in a single form comprising packets of bits of fixed length called cells. It is known to connect subscriber terminals to a telecommunication network using concentrators whose function is to concentrate calls from relatively lightly loaded subscriber circuits onto more heavily loaded multiplexes. Conversely, a concentrator takes calls from a heavily loaded multiplex and transfers them onto a line connected to a subscriber terminal which is relatively lightly loaded.
2. Description of the prior art
French patent application No 2 646 036 describes an all electronic digital subscriber access unit comprising a plurality of concentrators for connecting subscriber terminals using various transmission techniques, including asynchronous time-division multiplexing, to a telecommunication network. In these known type concentrators, each concentrator comprises subscriber access circuits and two concentrator stages.
Each subscriber access circuit provides functions including:
optical-electronic conversion and vice versa for each subscriber terminal connected to the concentrator by an optical fiber line;
delineation of the cells received from the subscriber terminal, by which is meant the identification of the start of each cell;
descrambling of the data field of each cell received from a subscriber terminal;
scrambling of the data field of each cell sent by the concentrator to a subscriber terminal;
calculation of the error detection word relating to the header of each cell sent from the concentrator to a subscriber terminal or received from a subscriber terminal;
resynchronization of the cells sent by a subscriber terminal to a local clock of the subscriber access unit;
a policing function which monitors the output bit rates of the subscriber terminals;
extracting signalling cells from the cells sent by a subscriber terminal and injecting signalling cells into the cells sent by the concentrator to a subscriber terminal;
conversion of the virtual circuit and virtual circuit group labels included in each cell sent by a subscriber terminal;
calculation of the routing label for routing each cell in the switching network and insertion of this label in front of the cell header;
elimination of empty cells sent by a subscriber terminal; and
insertion of empty cells into the cells sent from the concentrator to the subscriber terminals.
The first stage of concentration comprises an asynchronous time-division switching matrix controlled by a microprocessor. The second stage of concentration comprises another asynchronous time-division switching matrix or a time-division multiplexer-demultiplexer controlled by a microprocessor.
The design of this concentrator is optimized for use of electronic components but is not optimized for use of photonic components. These have the advantage of a higher speed and enable spectral multiplexing. Retaining the design of known type concentrators would result in underuse of the performance capabilities of photonic technology.
An object of the invention is to propose a photonic concentrator whose design is not merely deduced from that of a known electronic concentrator but which is optimized for the use of photonic components.